This program is concerned with studies on the development of parasitic helminths in vitro. Currently, it is focused on the following aspects: 1) the requirement of larval stages of nematodes for cholesterol for growth and differentiation. The cellular pathology resulting from the absence of cholesterol is being examined by light and electron microscopy. 2) We have found that various species of helminths concentrate very high levels of vitamin B12 whereas others do not. Attempts are being made to use this as a biochemical evolutionary marker and to correlate B12 concentration with particular metabolic pathways. BIBLIOGRAPHIC REFERENCES: Tkachuck, R.D., P.P. Weinstein, and J.F. Mueller. 1976. Comparison of the uptake of vitamin B12 by Spirometra mansonoides and Hymenolepis diminuta and the functional groups of B12 analogues affecting uptake. J. Parasitol. 62: 94-101.